This invention relates to the production of engineered abrasives on substrates in a form useful for grinding and finishing of substrates such as metals, wood, plastics and glass
The proposal to deposit generally isolated structures such as islands or ridges of a mixture of a binder and abrasive material on a backing material to form so-called xe2x80x9cengineered abrasivesxe2x80x9d, has been known for many years. If the islands or ridges have very similar heights above the backing and are adequately separated then, (perhaps after a minor dressing operation), use of the product will result in reduced surface scratching and improved surface smoothness. In addition the spaces between the islands provide a route by which swarf generated by the abrasion can be dispersed from the work area and coolant can circulate.
In a conventional coated abrasive, investigation of the grinding surface reveals that a comparatively small number of the surface abrasive grits in an active abrading zone are in contact with the workpiece at the same time. As the surface wears, this number increases but equally the utility of some of those abrasive grits may be reduced by dulling. The use of structured abrasives has the advantage that the uniform islands wear at essentially the same rate such that a uniform rate of abrasion can be maintained for longer periods. In a sense the abrading work is more evenly shared among a larger number of grinding points. Moreover since the islands comprise many smaller particles of abrasive, erosion of an island uncovers new, unused abrasive particles which are as yet undulled.
One technique for forming such an array of isolated islands or dots that has been described is that of the rotogravure printing. The technique of rotogravure printing employs a roll into the surface of which a pattern of cells has been engraved. The cells are filled with abrasive/binder formulation and the roll is pressed against a surface and the formulation in the cells is transferred to the surface.
Chasman et al. in U.S. Pat No. 4,773,920 disclosed that using a rotogravure coater, it is possible to apply a uniform pattern of ridges and valleys to the binder formulation which, when cured, can serve as channels for the removal of lubricant and swarf. However beyond the bare statement of possibility, no details are given that might teach how this might be carried out.
In U.S. Pat No. 4,644,703 Kaczmarek et al. used a rotogravure roll in a more conventional fashion to deposit an abrasive/binder formulation to deposit a layer that is then smoothed out before a second layer is deposited by a rotogravure process on top of the smoothed-out first layer. There is no teaching of the nature of the final cured surface.
In U.S. Pat No. 5,014,468 (Ravipati et al.) it was proposed to use an abrasive/binder mixture having non-Newtonian flow properties and to deposit this mixture by a rotogravure technique on to a film. In this process the mixture was deposited from the edges of the rotogravure cells to produce a unique structures with deposits of reducing thickness with distance away from the surface surrounding areas devoid of the mixture. If the cells are sufficiently close together, the surface structures can appear interlinked. This product has proved very useful, particularly in ophthalmic fining operations. A further refinement of such a rotogravure process was described in U.S. Pat No. 5,840.088. The process is very useful but it has a potential problem with increasing build-up of material in the cells of the rotogravure roll such that the deposition pattern can change slightly during a protracted production run. In addition the nature of the process is such that it is limited to formulations containing relatively fine abrasive grits, (usually less than 20 microns).
Another approach to making engineered abrasives is provided by depositing an abrasive/binder mixture on a substrate surface and then imposing a pattern comprising an array of isolated structures on the mixture by curing the binder while in contact with a mold having the inverse of the desired patterned surface. This approach is described in U.S. Pat Nos. 5,437,754; 5,378,251; 5,304,223 and 5,152,917. There are several variations on this theme but all have the common feature that each structure in the pattern is set by curing the binder while the composite is in contact with a molding surface.
In U.S. Pat No. 5,863,306 Wei et al. described another technique for making engineered abrasives by an embossing process applied to an abrasive /curable binder mixture.
U.S. Pat No. 5,833,724 (Wei et al.) refined engineered abrasive structures, deposited by any prior art technique, by the superposition of a xe2x80x9cfunctional powderxe2x80x9d over the engineered surface. This functional powder can be abrasive particles or a grinding aid or any other additive conveying a specific advantageous property on the engineered abrasive surface. Most often the powder is a mixture of abrasive particles and a grinding aid. Such a functional powder provides a very aggressive initial cut that is highly desirable.
The present invention provides an added improvement to this concept that ensures maximum benefit from the functional powder coating.
It has now been found that a coated abrasive can be made wherein the surface is engineered to comprise a plurality of shaped composites attached to a common backing material, said composites comprising a UV-cured resin with abrasive particles dispersed therein, and the surface of the shaped abrasive composites having a layer of particles of a functional powder adhered thereto characterized in that a top size coat overlies the functional powder particles.
The xe2x80x9ctop size coatxe2x80x9d is a layer comprising a cured binder which is deposited over the functional powder and acts to help retain the particles of powder in position during grinding. As the name indicates it is the topmost layer of the coated abrasive and is therefore the layer that first contacts a workpiece when the coated abrasive is in use. The top size coat can comprise other non-abrasive components such as a filler or a pigment to modify the physical properties and/or appearance of the surface. The binder can be a thermosetting resin or a radiation curable resin. Examples of such resins include phenol/formaldehyde resins; urea/formaldehyde resins; epoxy resins; (metharylate polymers and copolymers; urethane (meth)acrylate resins; polyester/(meth)acrylate resins; epoxy-meth)acrylate resins and other resins known in the art for such applications.
It is preferable that the top size layer is compatible with the layer over which it is applied. This is preferred to ensure that the cured top size layer will not flake off the layer immediately below under grinding conditions. For example composites in which the cured binder is an acrylate-based radiation-cured binder can be over laid by a top layer that is also an acrylate resin, an epoxy resin or a phenolic resin.
The invention is particularly useful when the engineered abrasive surface comprises a coating of a functional powder, separately applied and bonded to the surface or applied to the UV-curable binder/abrasive mixture from which the composites are formed before cure of the binder such that the powder is concentrated in the surface layer of the composites as taught in U.S. Pat No. 5,833,724.
In the present application the term xe2x80x9cfunctional powderxe2x80x9d is used to refer to finely divided material that modifies the abrasive qualities of the engineered abrasives to which it is applied. This can be as simple as making the engineered abrasive cut more aggressively or reducing the buildup of swarf or static charge on the surface. Some functional powders can additionally serve as a releasing agent or a barrier between the resin formulation and the embossing tool, reducing sticking problems and allowing improved release. Included under the heading of xe2x80x9cfunctional powdersxe2x80x9d are fine abrasive grits, grinding aids, anti-static additives, lubricant powders and the like. The individual particles of the powder typically have an average particle size, (D50), less than about 250 micrometers such as from 1 to 150 micrometers and more preferably from 10 to 100 micrometers.